1. Field of the Invention
This invention relates to subcutaneous power supplies having photovoltaic cells, or solar cells, and, more particularly to such power supplies that prevent the ingress of bodily fluids, and have high spectral response after insertion under living tissue.
2. Background Art
There are several commercially-available electronic devices implanted in humans and animals that require a power source, typically Lithium-Ion (L-I, one rechargeable type) or Silver Vanadium Oxide (SVO) batteries.
These devices include: cardiac pacemakers; heart defibrillators; pain suppressors; drug infusion pumps (typically for insulin), augmentation mechanical heart pumps, retinal eye implant devices, limb implant devices, bodily valve switches, which is a partial list. Also, implantable hearing aid devices, with other transmitter-receiver devices in the planning stage.
One interesting new bodily implanted transmitter-receiver(s) bridges the gap over severed or damaged nerves in living organisms. Researchers at Reading University, England have implanted a glass tube beneath the skin of a subject. The tube contains a device that transmits signals outside the body; it is a forerunner to nerve-gap transmission of nerve impulse signals. These devices presently employ a battery within the implanted device, charged by an internal-external induction coil. Although external-internal induction coils will work as a method of recharging implanted batteries, they are far from ideal as a charging method, because of the impracticality of positioning an external coil, and the inconvenience of the wearer remaining physically close to the external coil for very long periods of time, necessary to trickle-charge the batteries.
The trickle-charging method requires a rapid recharging process, which always shortens battery life. Very slow trickle charging of batteries at a rate not to exceed the discharge rate of the implant device they power is ideal for maximum battery longevity.
The average age of cardiac pacemaker batteries is seven years, and the average age of the recipient is seventy-two. The entire device may require replacement at age seventy-eight, since the battery is sealed inside. These batteries are permanently sealed inside laser welded titanium pacemaker cases, thus requiring the entire unit be replaced at great cost. However, at age seventy-eight, the wearer's health has frequently deteriorated to the point where they cannot withstand the trauma of replacement surgery, which results in death in 10%-15% of the cases. Teenagers requiring pacemakers or defibrillators could tolerate replacement surgery up to fifteen times during their lives.
The batteries powering heart defibrillators generally deplete more rapidly than pacemakers because they must maintain a constant charge in the unit's capacitors. When a firing sequence begins, the defibrillator must impart a strong electrical charge to shock the heart repeatedly, until a stabilized pattern of beats has been reestablished. This firing sequence must be repeated in rapid succession frequently, requiring a fast re-charging cycle of the unit's (usually one or two) capacitors. The sudden current demand on a defibrillator tends to be greater than that of pacemakers. Defibrillator batteries frequently last only three to six years due to the voltage requirement of the capacitors—around 400 volts.
Other implanted electrically-driven devices, such as the insulin infusion pumps, have a shunt leading into the pancreas, and another exiting the side of the abdomen, where it plugs into an external plastic bag of insulin. The bag is worn on a separate belt positioned above a patient's clothing belt, so that the shunt tube will not be pinched off. This internal pump includes a sensing device which measures the blood sugar level and switches the pump on and off, injecting one or two drops of insulin into the pancreas per-actuation. Trickle-feeding insulin prevents large fluctuations in the insulin level of the blood, which is presently the case when syringes are used.
Mechanical hearts require more electrical power, because their drive motors must open and close valves or spin centrifugal impellers, providing the propulsive force to push blood through the body against the body's natural resistance due to blood being progressively forced through narrowing capillaries. The difficulty of this task is increased when the wearer is standing or moving. While standing, blood must be pumped up to the brain, a level higher than the mechanical heart, and must be pumped to the feet and its return path up the veins to the lungs, where it is re-oxygenated.
The power supplied for such devices requires, ideally, one rechargeable battery inside or near the mechanical heart, and a much larger battery pack worn externally. These outside batteries typically comprise a series of thin units wired in a string and sewn into a belt-like construction with wires exiting the body and attaching to them, or by having a small flat inverter-induction coil attached to the inside wall of the battery pack-belt. This outside induction coil must be axially positioned over a second induction coil-inverter implanted underneath the skin—opposite the belt. This permits current to jump across the two coils, after it is first converted from d.c. to a.c. power, then penetrating the dermis, and entering the body as a.c. current, where it is reconverted back to d.c. for transmittal by wires to the mechanical heart.
Cardiac pacemakers presently account for the most widespread use of internal batteries, typically single cell L-I types, although newer batteries are being introduced. The L-I battery generates a nominal 2.8 volts from a single cell when fresh, and is allowed to drop as little as 0.2 volts before replacement is indicated. However, depending on the construction of the cathode and anode plates, the L-I battery can generate up to 3.7 volts from a single cell.
More efficient electrical circuit designs have been made through the years, however, the added telemetry functions for implantable devices have tended to offset these efficiency gains. With external telemetry, the surgeon can change the rate (time-duration-width) and the intensity (voltage) of the beats in a non-invasive manner. Regardless of the improvements in electronically implantable batteries and devices, the wearer will want as few replacement surgeries as possible.
What has been needed but heretofore unavailable, is an improved subcutaneous device for powering implantable medical devices of all kinds that is lightweight, flexible, has improved sealability, and has improved internal battery longevity.